Generally, the above type of front derailleur includes a chain guide, a base member fixed to a seat tube at the bicycle frame, a pair of linkage members pivoted to the base member, and a moving means for reciprocating the chain guide axially of the multistage chain gear assembly. Between the chain guide and the base member or one of two linkage members is interposed a return spring for biasing the chain guide in the direction of the backward movement. The base member screws with a pair of regulation screws for regulating a range of swinging motion of each linkage member, and in turn a range of swinging motion of the chain guide. The outer linkage member has an end which extends outwardly at the base member side. An operating wire is connected one end to a speed change operating lever and is mounted at its other end to the aforesaid extending end of the outer linkage member. The operating wire is pulled by an operating lever so that the chain guide moves forwardly against a spring force of a return spring and reaches the position corresponding to a high speed front chain gear, and the operating wire is released to return the chain guide by a restoring force of the return spring to the position corresponding to a low speed front chain gear.
The front derailleur sets the chain guide in position with respect to the front chain gear by use of a positioning means at a speed-change operating device such as a positioning mechanism provided between the operating lever and a member rotatable relative thereto and comprising a plurality of recesses and a rolling member engageable with one of the recesses, or by a display unit and indication unit.
However, when the operating wire is elongated by repeated speed-change operation by the lever the motion of the chain guide at the front derailleur becomes out of proper adjustment with respect to the movement of the lever, in other words, the amount of movement of the chain guide is insufficient with respect to the stroke the operating lever to place the chain guide at the desired gear, thereby creating a problem in that the driving chain cannot be switched to a desired front chain gear with accuracy.